1. Field of the Invention
The present invention relates to an iron nitride-based magnetic powder used in high-density magnetic recording media, and particularly to one that has superior weatherability such that the deterioration over time of its magnetic properties is mitigated, and to a method of manufacturing the powder.
2. Background Art
It is desirable for recent magnetic recording media to have high recording densities, and in order to achieve this goal, the recording wavelengths are becoming shorter and shorter. Unless the magnetic particles are of a size considerably smaller than the length of a magnetic domain recorded by means of a short-wavelength signal, recording becomes effectively impossible since a clear magnetization transition state cannot be created. Thus, the magnetic powder is required to have a particle size much smaller than the recording wavelength.
In addition, in order to achieve higher recording densities, the resolution of the recording signal must be increased and to this end, noise in the magnetic recording medium must be reduced. Noise is greatly affected by the particle size, with noise becoming lesser the finer the particle. Accordingly, magnetic powders for use in high-density recording are required to have very small particle sizes on this point also.
However, as particles become finer, it becomes more and more difficult for the particles to remain present as single independent particles, and there is a problem in that even in the case of the metal magnetic powder typically used for data storage, if the particle size becomes extremely fine, sintering readily occurs during reduction in the manufacturing process. If sintering occurs, then the particle volume becomes large and this becomes a source of noise, leading to deleterious effects such as deterioration in dispersibility and a loss of surface smoothness when made into tape. Magnetic powder suitable for high-density recording media must have good magnetic properties as a magnetic material, but in addition, when being made into tape, its powder properties, namely the average grain size, grain-size distribution, specific surface area, TAP density, dispersibility and the like become important.
Up until now, iron nitride-based magnetic powders with a Fe16N2 phase as the main phase have been known as magnetic powders suited for high-density recording media that have superior magnetic properties, as disclosed in JP 2000-277311A (Patent Document 1) and WO 03/079333 A1 (Patent Document 2). For example, Patent Document 1 discloses an iron nitride-based magnetic substance with a large specific surface area that exhibits a high coercivity (Hc) and high saturation magnetization (σs), teaching that the synergistic effect of the magnetocrystalline anisotropy of the Fe16N2 phase and the increased specific surface area of the magnetic powder allows high magnetic properties to be obtained regardless of the shape morphology. Patent Document 2 recites a magnetic powder that is improved over that of Patent Document 1, being a magnetic powder that substantially comprises a spherical or oval magnetic powder of rare earth-iron-boron, rare earth-iron, or rare earth-iron nitride, teaching that if tape media are fabricated using these powders, then superior properties are obtained. Among these powders, despite being fine particles on the order of 20 nm, the rare earth-iron nitride-based magnetic powder with the Fe16N2 phase as the main phase has a high coercivity of 200 kA/m (2512 Oe) or greater, and the specific surface area found by the BET method is small, so the saturation magnetization is high and its storage stability is also good. It is recited that by using these rare earth-iron nitride-based magnetic powders, the recording density of coating-type magnetic recording media can be dramatically increased.
The method of manufacturing these rare earth-iron nitride-based magnetic powders is an ammonia nitriding method wherein: the rare earth-iron nitride-based magnetic powder is formed by reducing particles of magnetite with a rare earth and one or both of Al or Si adhered to the surface of the particle, and then nitriding with NH3 gas. Because of the large magnetocrystalline anisotropy of the Fe16N2 phase induced by this nitriding, it is possible to obtain magnetic powders suited to high-density recording media, or namely magnetic powders consisting of fine particulates that have high Hc, high σs and other properties.
However, as recited in Patent Documents 1 and 2, magnetic powders containing the Fe16N2 phase that have both a small average grain size and superior magnetic properties have been demonstrated to have good potential as magnetic materials, but nothing is disclosed regarding their properties as powders, e.g., their grain size distribution, dispersibility and the like, so it is difficult to determine whether or not they are magnetic powders suitable for the coating-type magnetic recording media used. Even magnetic powders with superior magnetic properties, if they bring the tape to poor surface smoothness, for example, would ultimately not be suitable for use in coating-type magnetic recording media.
In Patent Document 2, at the time of producing the Fe16N2 phase that has a large magnetocrystalline anisotropy, Si, Al and rare earth elements (including Y) are adhered to the particle surface so as to produce fine particles that do not undergo sintering. However, with this method of preventing sintering by adhesion, in the case that the conditions for adhesion are inadequate, the degree of adhesion of the sintering-preventative agent may be different for each particle, so there may be places where sintering is prevented where adhesion is adequate and places where sintering occurs where adhesion is poor. As a result, there is a problem in that the grain size distribution of the powder thus obtained is poor. In fine particles in particular, the particles agglomerate readily and tend to behave as an aggregate, so uneven adhesion readily occurs. A poor grain size distribution may cause deterioration of the tape surface properties, or even deterioration of the electromagnetic transduction properties.
As a result of various studies conducted by the present inventors in order to solve these problems, the inventors discovered that if goethite in solid solution with Al is used as the starting material for the manufacture of iron nitride-based magnetic powder, then one can obtain an iron nitride-based magnetic powder constituted primarily of Fe16N2 that has superior magnetic properties suited to high-density magnetic recording media, a narrow grain size distribution, fine particles with an average grain size of 20 nm or less that do not sinter and good dispersibility when made into tape, and thus the inventors filed Japanese patent application number 2004-76080.